Effects of thermal processing combined with adding ascorbic acid on the quality of sour watermelon juice

Abstract:【Objective】Watermelon is one of the most popular fruits in summer because it has an exotic flavor and is a nutrient-rich fruit, with abundant lycopene, carotene, sugars, vitamins, minerals, amino acids, et al. Watermelon juice is one of important products of watermelon. However, watermelon is a heat-sensitive fruit, heating and sterilization could lead to browning, precipitation and strong“cooked taste”of the watermelon juice, and seriously affect the sensory quality. The research on quality control technology of watermelon juice before and after hot processing is particularly important for the production of watermelon juice. A series of sour watermelon varieties have been bred by Zhengzhou Fruit Research Institute, Chinese Academy of Agricultural Sciences. There has been little research on the processing of sour watermelon. It is necessary to investigate the adaptability and thermal processing technology of watermelon juices from different varieties of sour watermelon.【Methods】In this study, 6 varieties of sour watermelon were used as raw materials to make juice The selected sour watermelon juices were treated with thermal processing and ascorbic acid. The color, sensory quality and flavor substances of watermelon juices before and after thermal processing were analyzed.【Results】From the perspective of sensory evaluation, the quality of the fresh processed and hot processed watermelon juice made by SW-20, SW-27 and SW-14 were excellent. The sensory quality of the yellow flesh watermelon juice was better than that of the red watermelon juice. The brightness L*, color saturation C* and hue angle h of 2 color watermelon juice decreased or significantly decreased (p＜0.05) after thermal processing. The b*, C*and h of watermelon juice added with ascorbic acid (Vc) were significantly higher than those without Vc (p < 0.05) before and after heat treatment. The addition of Vc contributed to maintaining the color of the red fresh processed watermelon juice and the hot processed watermelon juice. The color, form, taste, odor and sensory total scores of the hot processing watermelon juice added with Vc were significantly higher than those without Vc (p < 0.05). The flavor substances of the watermelon juices of two color varieties SW-14 and SW-20 were identified by headspace solid phase micro-extraction and gas chromatogra- phy-mass spectrometry technology. 47 flavor compounds of the watermelon juice were identified before and after hot processing. The types and contents of alcohols and aldehydes were the highest. Among them, 2, 6-nonenol, 3-nonenol, 6-nonenal and 2, 6-nonenal were the main components. The thermal pro- cessing and the addition of Vc had great influence on the types and contents of watermelon juice flavor substances. There were 41 kinds of flavor substances in the juice of SW-14 without treatment of Vc and 30 kinds of flavor substances in the juice of SW-14 with Vc treatment, and after hot processing, there were 46 kinds and 40 kinds of flavor substances in the above two juices. The flavor substances in the four kinds of watermelon juices of SW-20 were 34, 30, 39 and 31, respectively. This indicated that hot processing increased the variety of flavor substances in the watermelon juice, while adding Vc reduced the variety of flavor substances in the watermelon juice. The proportion of furans and 6-nonenal leading to bad odor in the watermelon juice increased after hot processing, while the proportion of 2, 6-nona- dienol and 3-nonenol leading to sweet cucumber odor decreased. The proportion of furans and 6-none- nal leading to bad smell in the watermelon juice with Vc treatment decreased, while the proportion of 2, 6-nonadienol and 3-nonenol increased.【Conclusion】The watermelon varieties SW-20 and SW-14 were suitable for making juice. Thermal processing and adding Vc had great influence on the quality and fla- vor of the watermelon juice. Adding Vc could help to maintain the color of watermelon juice, reduce the cooking odor of the watermelon juice after hot processing, and improve the sensory quality. This study would provide theoretical basis and technical reference for the processing of watermelon juice.